Using your method part a) comes to 1.5/0.4=3.75m/s^2.Your method is right but something went wrong with your maths(used calculator perhaps and pressed wrong buttons?).Please note that you could have used the whole triangle and written a=3/0.8.

You made a similar mistake in part c)

You need to improve on your reasoning for part b).The acceleration is zero because it is a straight line parallel to the time axis,in other words there is no change of velocity.